The present invention relates to jet aircraft exhaust nozzles and, more particularly, to methods of design and the construction of a supersonic nozzle that has a fixed exit area, a shallow boat tail and a shock free operation at the designed pressured ratio.
A propelling nozzle is the component of a jet engine that operates to constrict the flow, to form an exhaust jet and to maximize the velocity or propelling gases from the engine. Propelling nozzles can be subsonic, sonic, or supersonic. Physically the nozzles can be convergent, or convergent-divergent. Convergent-divergent nozzles can give supersonic jet velocity within the divergent section, whereas in a convergent nozzle the exhaust fluid cannot exceed the speed of sound within the nozzle.
Propelling nozzles can be fixed geometry, or they can have variable geometry, to give different throat and exit diameters so as to deal with differences in ambient pressure, flow and engine pressure; this permitting improvement of thrust and efficiency.
A propelling nozzle operates by using its narrowest part, or “throat”, to increase pressure within the engine by constricting airflow, then expanding the exhaust stream to, or near to, atmospheric pressure, and finally forming it into a high speed jet to propel the vehicle.
The energy to accelerate the stream comes from the temperature and pressure of the gas, the gas cools, expands, and accelerates, with the heat and pressure of exhaust gas being proportional to its speed.
Air-breathing engines create forward thrust on the airframe by imparting a net rearward momentum onto the air via producing a jet exhaust gas, which, when fully expanded, has a speed that exceeds the aircraft's airspeed.
Engines that are required to generate thrust quickly from idle use propelling nozzles with variable area. While at idle, the nozzle is set to its open configuration for minimum thrust and high engine rpm, but when thrust is needed, e.g. while initiating a go-around, constricting the nozzle will quickly generate thrust.
Almost all nozzles have a convergent section because it increases the pressure in the rest of the engine-potentially yielding more thrust by acting on the forward sections. It is important to note that convergent nozzles end with this convergent section, and in general, narrower convergent nozzles give lower thrust and higher exhaust speed, but wider convergent nozzles give lower exhaust speed and higher thrust.
Simple convergent nozzles are used on many jet engines. If the nozzle pressure ratio is above the critical value of 1.8:1, a convergent nozzle will choke, resulting in some of the expansion to atmospheric pressure taking place downstream of the throat, i.e. smaller flow area, in the jet wake. Although jet momentum still produces much of the gross thrust, the imbalance between the throat static pressure and atmospheric pressure still generates some pressure thrust.
The high nozzle pressure of convergent nozzles often cause the pressure of exhaust exiting the engine to exceed the pressure of the surrounding air and thereby reduce efficiency by causing much of the expansion to take place downstream of the nozzle itself. Consequently, some engine, e.g. rockets, incorporates a convergent-divergent nozzle which cause more of the exhaust to expand against the inside of the nozzle.
Engines for jet aircraft may be constructed as fixed exhaust nozzle systems and/or variable exhaust nozzle systems. Fixed exhaust nozzle systems are commonly used on varies types of commercial aircraft, and some military aircraft. Variable exhaust nozzle systems are commonly used on supersonic military aircraft, and which allow for kinetically changing the shape of the nozzle to accommodate for different thrust levels and other factors. While such variable exhaust nozzle systems provide certain advantages, they also introduce added complexity, expense, and in some cases, may require compromises in other areas of operation.
Fixed exhaust nozzle systems are commonly designed for specific thrust levels to make them very efficient. However, fixed exhaust nozzle systems may also be designed to define flow paths to accommodate changes in thrust levels without the need of added expense and complexity of the variable exhaust nozzle systems. The present invention is directed to such a fixed exhaust nozzle system.
Fixed nozzle designs, as described herein, include single expansion ramp nozzles (SERNs) that partially expand the exhaust gas internally, within the nozzle. The remainder of the exhaust gas expands external to the SERN.
A properly designed SERN typically allows the exhaust gas to automatically expand according to the ambient pressure, without using a variable of the geometric nozzle. SERNs are desirable for applications where nozzle pressure can vary widely throughout the flight envelope. However, SERN nozzles require a specific flow angle (dictated by the Prandtl-Meyer Theory), where the throat plane is able to achieve a thrust vector angle of zero at the design pressure ratio. This angle is often steep relative to the direction of flight. As a result the boat tail angle of the outer mold line (OML) at the last enclosed area of the exhaust duct (i.e. the throat plane of a SERN) is steep. The drag penalty associated with this arrangement is severe enough to obviate a use of a SERN in thrust-drag optimized nozzle configuration.
To reduce the flow angle required at the throat of the SERN, and reduce the boat tail angle, some of the flow expansion can be undertaken within the SERN. The angle of the partially expanded flow is much shallower compared to a conventional SERN. The remainder of the expansion is undertaken by an external ramp. This type of arrangement is referred to as an nxSERN, which owes its name to the unique internal/external arrangement of isentropic geometries. All of the advantages embodied in the conventional SERN are retained in the nxSERN concept, with the added benefit of reduced boat angle.
U.S. Pat. No. 3,146,584 (Jet Propulsion Nozzle) describes a conventional SERN nozzle as described above. While such a nozzle takes advantage of the CFG performance benefit provided by a SERN, it presents integration challenges due to the boat tail angle issues described above. It also provides no means for integrating a third stream exhaust without disrupting the main exhaust flow.
U.S. Pat. No. 6,948,317 (Methods and Apparatus for Flade Engine Nozzle) ('317 Patent) is also representative of a conventional SERN. While the boat tail issues associated with SERN are alleviated, in the disclosed design, the CFG performance is compromised. Additionally, the provision for a third stream exhaust still disrupts the flow even though the provision is built into the design. Moreover, the internal surface of the outer flap as described in the '317 Patent is an undefined curve that can degrade CFG performance at any pressure ratio. Similarly, the “ramp flap” and “flade flap” shown in the '317 Patent are undefined curves that are not optimized for CFG performance.
While much is known regarding the design of a fixed exhaust nozzle system, engine manufactures have generally lacked the capability to design a conceptual nozzle flow path from a compressible flow first principles. Instead, such manufacturers have been commonly used a design process starting with a preconceived nozzle flow path, fine-tuned by the integrated process of experimentation, followed by CFT analysis to fine-tune the design concepts. This brute force approach is time consuming and leaves the manufacturers with no clear method of achieving an optimized solution. The introduction of the nozzle design methodology, defining a flow path for compressible flow first principles is desirable to allow nozzle designs to proceed from more precise initial concepts, even if the nozzle designs are subsequently tailored for optimization in view of the other criteria, e.g. low observable characteristics.